The present invention relates to a digital FM modulating apparatus and, more particularly, to a digital FM modulating apparatus used for transmitting digital data such as a television signal and an audio signal.
As shown in FIG. 1, a conventional digital FM modulating apparatus comprises an A/D converter 1, a multiplier 2 for multiplying a digital modulation signal by multiplication data designating the maximum deviation frequency, an adder 3 for adding the output of the multiplier 2 with addition data designating a center frequency and a direct digital synthesizer (DDS) 4. Further, the DDS 4 includes a reference oscillator 5 for generating sampling timings, an adder 6, a latch 7, a read-only memory (ROM) 8 for storing sine wave data and a D/A converter 9. In the DDS 4, the incoming data from the adder 3 are accumulated through the latch 7 at the sampling timings and the accumulated data from the latch 7 are applied to the ROM 8 as address data. Accordingly, FM modulated signal data are obtained from the ROM and, then, converted into analog FM modulated signal by the D/A converter 9.
However, a conventional digital FM modulating apparatus has the following drawbacks. When a digital data error or abnormal data occurs through data transmission to the ROM 8, the resultant modulated signal has an undesirable spread frequency spectrum beyond the regular range. Further, when an erroneous digital data transition occurs through data transmission, the frequency spectrum is also undesirably spread.